Fix Issue 21220 - [DIP1000] scope variable may escape through scope dynamic array parameter

[RazvanN7]

@safe:

struct A
{
    char[] value;                                                                                                                             
}

A f(scope A[] array)
{
    return array[0]; //  this line here escapes the `value`
}

It seems that [1] introduced this behavior on purpose, but I don't understand why. If array is made into a static array then returning array[0] is going to issue an error. However, it does not make any sense to me that the behavior is changed for dynamic arrays.

cc @WalterBright

[1] #8030

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21220	regression	[DIP1000] scope variable may escape through scope dynamic array parameter

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

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Testing this PR locally

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dub run digger -- build "master + dmd#12204"

[PetarKirov]

I'm afraid that it is by design - scope is not transitive.
I haven't checked, but I expect that replacing the array slice with a pointer to have the same behavior (as it should, as slices have pointers). Static arrays are fundamentally different - they're embedded inside the parent object, and so struct S1 { T x, y, z; } is equivalent to struct S2 { T[3] xyz; } (as far as object lifetime is concerned).

That said, there's plenty of cases where scope should be transitive, so I don't agree with that aspect of the design.

[Geod24]

I'm afraid that it is by design - scope is not transitive.

That's the gist of it. I closed the issue as a duplicate of 20505.
There's also a PR for this: #10951
And yeah, the lack of transitivity really makes it unusable (e.g. #8369 ).